Shiyou shiyan dizhi (Mar 2021)
Characteristics of in situ stress of tight oil reservoirs and its influence on petrophysical properties: a case study of Upper Triassic Yanchang Formation in Ordos Basin
Abstract
The petrophysical properties of tight reservoirs and the state of fluid occurrence are affected by current geostress. In the past, there were few studies on the rock mechanical properties and in situ stress of the Yanchang Formation in the central and western regions of the Ordos Basin, which restricted the efficient exploration and development of tight oil and gas. A systematic study was carried out on the rock mechanical properties, in situ stress characteristics and their effects on the petrophysical properties of the Chang 6-Chang 8 reservoirs in the Wuqi, Zhidan and Dingbian areas in the Ordos Basin. A reliable method of in situ stress logging interpretation using fracturing methods and logging models was established. The Chang 6 to Chang 8 in Wuqi area is not significantly stressed. The horizontal stress activity deep in the Zhidan area is greater than that in the shallow layers, with the strong horizontal stress intensity mainly concentrated in Chang 73 and Chang 8. The horizontal stress activity of the Dingbian area is relatively strong from Chang 6 to Chang 8, and the horizontal stress intensity of the shallow layer is slightly stronger than that of the deep layer. The stress gradients in the three work areas are determined. The horizontal principal stress gradient gradually increases from northwest to southeast. Finally, the influence of horizontal compression stress on the path of rock compaction and the petrophysical properties of the reservoir are systematically discussed. It was found that the increase of σH-σh did not cause the loss of rock pores alone. The stress plane heterogeneity will cause three different paths of rock porosity changes. However, the increase of the horizontal principal stress difference will mainly cause a decrease of rock permeability.
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